Evidence indicates that increased production of IL-6 and its receptor are pathogenetic factors for bone loss associated with either PTH excess or loss of sex steroids. Moreover, sex steroids inhibit the expression of IL-6 and its receptor signaling component, gp130, whereas both genes are up-regulated by PTH. It is not known if these hormones regulate the transcription of the IL-6 gene in vivo, nor is it known if the opposing actions of estrogen and PTH on bone result, in part, from their opposing effects on IL-6 and/or gp130 expression. In addition, the cellular source of IL-6 production in estrogen deficiency or PTH excess has not been determined. The ultimate goals of this application are to elucidate the in vivo regulation of the IL-6 and gp130 genes by estrogen and PTH, to identify the cellular source of IL-6 in estrogen deficiency and PTH excess, and finally to obtain direct evidence that the opposing regulation of these two genes by the two hormones underlies their opposing effects on bone. Preliminary studies show that modulation of functional gp130 levels by stable transfection with dominant negative gp130 alter the magnitude of gp130 signaling in response to IL-6 and soluble IL-6 receptor (sIL-6r). Transgenic mice harboring an IL-6 promoter-luciferase construct have also been generated. The cellular models will be used to test the hypothesis that the opposing effects of estrogen and PTH on gp130 expression lead to corresponding changes in the responsiveness of bone cells to IL-6 type cytokines. Moreover, the transgenic mouse model will be used to test the hypothesis that estrogen and PTH have opposing effects on IL-6 transcription in vivo in specific cell populations, and that these effects are mediated by specific regulatory elements in the IL-6 promoter. Specific aspects of these hypotheses will be addressed by determining if changes in gp130 levels, similar to those observed after estrogen loss or PTH treatment, alter stromal/osteoblastic cell support of osteoclastogenesis in response to IL-6 type cytokines. The transgenic mice will be used to quantitate and localize IL-6 gene expression after estrogen loss, PTH infusion, or a combination of both. Finally, cis-acting regulatory elements in the IL-6 promoter that are the targets for estrogen and PTH actions in vivo will be identified.